Women who smoke during pregnancy may be putting their children at increased risk for conduct problems in childhood, researchers found.

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A significant association between maternal smoking during pregnancy and offspring conduct problems was observed in several studies.

The association was seen both among children reared by genetically-related mothers and those raised by genetically-unrelated mothers.

Women who smoke during pregnancy may be putting their children at increased risk for conduct problems in childhood, researchers found.

Across three different studies, children whose mothers smoked during pregnancy -- whether they were reared by their genetically-related mother or an adoptive mother -- had higher mean scores on measures of conduct problems compared with children whose birth mothers did not smoke during pregnancy, according to Gordon Harold, PhD, of the University of Leicester in England, and colleagues.

In the two studies that included children raised by their genetically-related mothers, there was a positive association with number of cigarettes smoked per day and conduct problem scores among genetically-related children (P<0.001 and P=0.005), they wrote online in JAMA Psychiatry.

In the two studies (including one from the previous pair) that included children adopted at birth and raised by genetically-unrelated mothers -- but whose birth mothers smoked during pregnancy -- researchers found a positive association between conduct problem scores and the number of cigarettes that their genetically-related mothers smoked daily while pregnant (P=0.007 and P=0.04).

Prior studies have shown a relationship between children's conduct disorders and prenatal smoking in mothers, although that research did not separate prenatal environmental influences from genetic and postnatal environmental factors.

The authors studied the relationship between genetically-related mothers' smoking while pregnant with their children's conduct disorders in three studies of 1,088, 310, and 636 children, respectively, raised by genetically-related or -unrelated mothers.

The first study -- the Christchurch Health and Development Study -- was a longitudinal birth cohort in New Zealand that assessed maternal smoking during pregnancy and child behavior until age 7.

The second study -- the Early Growth and Development Study -- was a U.S., ongoing, longitudinal, multisite study of adopted children and parents, as well as birth parents. Birth parent data were used to assess maternal smoking, while adoptive parent data were used to assess the child's home environment.

The third study -- the Cardiff In-Vitro Fertilization (IVF) study -- was an English study of children conceived through reproductive technologies such as homologous IVF, single-parent IVF, and embryo donation, where authors focused only on children born through maternal IVF.

Conduct problems were reported by mothers and teachers in the Christchurch study, by adoptive mothers and fathers in the Early Growth study, and by mothers and fathers in the IVF study.

Parenting practices were assessed through measures of maternal emotional responsiveness and avoidance of restriction, and punishment and hostility.

Maternal smoking habits were broken down into zero cigarettes daily, one to nine cigarettes daily, and 10 or more cigarettes daily.

The prevalence of smoking varied by study. The New Zealand study had a prenatal smoking prevalence of 50% among children raised by genetically-unrelated mothers and 32.7% by those raised by their birth mothers. In the U.S. sample, smoking was common to 40.8% of pregnant mothers. The English study had a prevalence of 5.7% of smoking mothers who raised their children, and 3.9% of smoking mothers whose children were raised by others.

Although rates of conduct problems differed between studies, associations between maternal smoking and child behavior problems were significant in each. However, there was no significant relation between maternal smoking and behavior issues in children born through embryo donation (P=0.98).

An accompanying editorial by Theodore Slotkin, PhD, of Duke University Medical Center, noted that "There is little doubt that at least one component of tobacco smoke, nicotine, is sufficient to disrupt brain development and to evoke subsequent behavioral abnormalities" in prior research, and that the current study clearly shows that the effects seen in animal models are replicated in humans.

The model used in this literature should inform future studies into toxicants responsible for neurodevelopmental disorders related to cigarette smoke, Slotkin added.

The authors found their study was limited by the small number of genetically-unrelated smokers in the English study, as was the prevalence of maternal smoking during pregnancy among those genetically-related mothers. The study was also limited by a lack of data on co-exposure to other drugs or alcohol, use of self-reported smoking data, and lack of time-specific data on smoking and other behaviors and exposures.

The study was supported by the Health Research Council of New Zealand, the Vanterbury Medical Research Foundation, the Child Health Research Foundation, the New Zealand Lottery Grants Board, the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the National Institute on Drug Abuse, the Office of Behavioral and Social Sciences Research, the National Institutes of Health, the U.S. Public Health Service, the National Institute on Drug Abuse, the National Institute of Mental Health, the Office of Behavioral and Social Sciences Research, the Wellcome Trust, and the Nuffield Foundation.

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